The present invention relates to the manufacture of image sensor die on a semiconductor wafer, and more particularly, towards an image sensor structure that has increased yield.
Image sensors are electronic integrated circuits that can be used to produce still or video images. Solid state image sensors can be either of the charge coupled device (CCD) type or the complimentary metal oxide semiconductor (CMOS) type. In either type of image sensor, a light gathering pixel is formed in a substrate and arranged in a two-dimensional array. Modern image sensors typically contain millions of pixels to provide a high resolution image. An important part of the image sensor are the color filters and micro-lens structures formed atop of the pixels. The color filters, as the name implies, are operative, in conjunction with signal processing, to provide a color image. The micro-lenses serve to focus the incident light onto the pixels, and thus to improve the fill factor of each pixel.
In general, the manufacturing process of integrated circuit (IC) image sensors is similar to that of the vast majority of ICs. A relatively large semiconductor wafer (for example, 300 mm diameter) proceeds through a number of deposition, etching, masking, cleaning, and other steps in order to form devices onto the wafer. Moreover, it is not uncommon for hundreds, if not thousands of discrete IC die to be formed on a single wafer. When the IC die have been fully formed, the wafer is sliced (referred to as dicing) along scribe lines formed in the wafer to separate the individual IC die from one another. After the individual IC die have been separated, the IC die are then mounted onto a package using a die bonding process.
The process of dicing the wafer invariably will result in some amount of particulate matter to be generated that is undesirably deposited onto the micro-lens surface of the IC die. While a cleaning step (using a spray) is typically performed to remove the particulates, this is still not sufficient to remove all of the particulates. This results in a reduced yield.
Moreover, because the micro-lens surface is generally left exposed, a special handling mechanism was employed to prevent damage to the image sensor. The special handling mechanism typically would involve handling the IC die only be the sides and not the micro-lens surface. This in turn further complicated the die bonding process between the IC die and the IC package.